Earth and the atmosphere

The Earth's structure

The Earth is made up of three main layers:

• Core (inner: solid iron/nickel; outer: liquid iron/nickel) — the source of Earth's magnetic field.
• Mantle — semi-solid rock that flows very slowly due to convection currents driven by heat from radioactive decay.
• Crust — thin solid rock layer. Continental crust is thicker (~30–70 km, less dense granite); oceanic crust is thinner (~5–8 km, denser basalt).

Tectonic plates are sections of crust (and upper mantle) that move due to convection currents in the mantle. Evidence for plate tectonics:

• Matching coastlines (e.g. South America and Africa)
• Similar fossils on continents now separated by oceans
• Matching rock types and mountain chains across ocean basins
• Evidence from seismic wave analysis

Continental drift: Alfred Wegener proposed this theory in 1912, but was not accepted for decades because he could not explain the mechanism. The mechanism (mantle convection) was understood later when seafloor spreading was discovered.

Evolution of the atmosphere

Current atmosphere (approx.): 78% nitrogen (N₂), 21% oxygen (O₂), 1% argon, 0.04% CO₂, plus water vapour (variable).

Early Earth atmosphere (~4 billion years ago): mainly CO₂, water vapour, nitrogen; little or no oxygen. Similar to the atmosphere of Venus and Mars today.

How the atmosphere changed:

1. Oceans formed as water vapour condensed. CO₂ dissolved in the oceans and formed carbonate rocks (limestone) — removing CO₂.
2. Photosynthesis by early bacteria/algae released oxygen into the atmosphere: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
3. Oxygen accumulated over billions of years; ozone (O₃) layer formed in the stratosphere, protecting life from UV radiation.
4. Nitrogen accumulated (relatively unreactive — not removed by most processes).

The greenhouse effect and climate change

Greenhouse gases (GHGs): CO₂, methane (CH₄), water vapour, nitrous oxide (N₂O). They absorb infrared radiation emitted by Earth's surface and re-emit it in all directions — including back towards Earth — warming the planet.

The natural greenhouse effect is essential for life (without it, Earth's average temperature would be about −18 °C). The enhanced greenhouse effect from human emissions is driving global warming.

Human activities increasing GHGs:

• Burning fossil fuels (coal, oil, natural gas) → CO₂
• Deforestation → less CO₂ absorbed by photosynthesis
• Agriculture (cattle, rice paddies) → CH₄
• Landfill → CH₄

Evidence for climate change:

• Rising global average temperatures
• Melting ice caps and retreating glaciers
• Rising sea levels
• Shifting seasons and extreme weather events

Sustainability and reducing carbon footprint

Strategies:

• Renewable energy (solar, wind, tidal, hydroelectric) to replace fossil fuels
• Carbon capture and storage (CCS)
• Electric vehicles
• Improved insulation in buildings
• Reforestation / afforestation
• Carbon taxation and international agreements (Paris Agreement)

Life cycle assessment (LCA): evaluates the environmental impact of a product from raw material extraction to disposal. Used to compare materials (e.g. glass vs plastic bottles) for sustainability decisions.

WJEC examiner requirement: evaluate the evidence for climate change and discuss competing factors. Be prepared to argue both the scientific consensus view and describe challenges/uncertainties.

Common examiner traps

1. Greenhouse effect vs enhanced greenhouse effect: the natural effect is essential and beneficial; the enhanced effect (increased GHG concentration from human activity) is the problem.
2. Greenhouse gases absorb IR, not UV: they absorb infrared (heat) radiation emitted by Earth's surface. The ozone layer absorbs UV from the Sun — a different mechanism.
3. Oxygen did not come from volcanoes: it came from photosynthesis by early cyanobacteria/algae.
4. Wegener's theory was initially rejected because he couldn't explain the mechanism. The mechanism is mantle convection driving plate tectonics.